The invention relates to a jig saw equipped with a cam arrangement for developing a pendular movement of the saw-blade holder and saw blade.
It has been long known that it is advantageous if the saw blade or saw-blade holder is caused to traverse a closed path such as an elliptical path. This has been achieved by imparting to the saw blade or saw-blade holder a reciprocating movement in a direction transverse to the up and down reciprocating movement of the saw-blade holder. The transverse movement resulting in the orbital motion has been achieved in prior art devices by actuating the shaft or blade holder in the horizontal direction usually with the aid of a roller mounted on a lever such as a bell crank and actuated by the drive means of the tool. Still another arrangement calls for a reciprocating cam to impart a transverse movement to the saw-blade holder.
One such prior art apparatus is disclosed in U.S. Pat. No. 3,890,708 wherein a balancing weight is configured as an elongated member and is driven by an eccentric separate from the eccentric driving the saw-blade holder. The balancing member has a cam surface formed at its lower end which engages a roller mounted on a frame holding the saw-blade holder. The frame is resiliently biased toward the cam surface of the balancing member so that the reciprocating movement of the balancing member produces a lateral reciprocating movement of the frame and therefore of the saw-blade holder. That is, a supplementary movement in the feeding direction of the saw-blade is achieved. Means are also provided for adjusting the stroke of the lateral reciprocating movement.
This known jig-saw is not only expensive to construct since in addition to the usually eccentric drive for the saw-blade holder, a supplementary eccentric drive is needed for achieving the upward and downward movement of the balancing member. And this balancing member also needs an additional guide arrangement so that it will not deviate from its pre-determined upward and downward movement. Furthermore, the desired form of the pendular movement is not achieved. As already mentioned, the balancing member also constitutes the balancing weight and it is therefore necessary that the balancing member be reciprocated in exactly 180.degree. phase-angle relation with respect to the saw-blade holder. Therefore, the balancing member is moved to its highest position when the saw-blade holder is at its lowest position and vice versa. This arrangement causes the saw-blade to move along an inclined path. However, it is not a true pendular movement wherein a specific point on the saw-blade traces out an enclosed path.
Jig saws are also known wherein the lateral movement for achieving pendular motion is achieved with the aid of a bell-crank lever actuated by a drive. The bell-crank engages the saw-blade to move the same in the horizontal direction while at the same time a scotch-yoke mechanism causes the blade to reciprocate in the upward and downward direction. In this kind of arrangement, the transverse movement is imparted directly to the saw-blade by means of a roller at one end of the bell-crank lever. The roller is situated such that the waste chips cut out by the saw blade from the workpiece will get between the saw blade and the roller and contribute to a wearing of the roller. In addition, this construction is expensive and requires a large number of individual parts so that assembly costs are high.
There are still other arrangements for achieving pendular motion of the saw-blade which likewise use a roller. For example, the transverse motion is achieved by configuring the back end-face of the saw-blade as a cam surface and as this cam surface rides on the roller, a movement in the transverse direction is achieved. This arrangement has the disadvantage that specially made saw-blades are needed in lieu of standard saw-blades. Further, an adjustment of the orbital loop is achieved only by installing a saw blade having a different cam surface formed on its rear end-face.